Background: D-VTd plus ASCT was approved for transplant-eligible (TE) NDMM based on part 1 of CASSIOPEIA. We report a prespecified interim analysis of CASSIOPEIA part 2: DARA maintenance vs OBS in pts with ≥partial response (PR) in part 1, regardless of induction/consolidation (ind/cons) treatment. Methods: CASSIOPEIA is a 2-part, randomized, open-label, phase 3 study in TE NDMM. Pts received 4 cycles ind and 2 cycles cons with D-VTd or VTd. 886 pts who achieved ≥PR were rerandomized to DARA 16 mg/kg IV Q8W for up to 2 yr (n = 442) or OBS (n = 444) until progressive disease per IMWG. Pts were stratified by ind (D-VTd vs VTd) and depth of response (minimum residual disease [MRD] status and post cons response ≥PR). Primary endpoint was progression-free survival (PFS) after second randomization. This interim analysis assessed efficacy and safety after 281 PFS events. A preplanned hierarchical procedure tested key secondary endpoints: time to progression (TTP), ≥complete response (CR), MRD negativity rates by NGS and overall survival (OS). Results: At median follow-up of 35.4 mo, median PFS was not reached (NR) with DARA and 46.7 mo with OBS (HR 0.53; 95% CI 0.42–0.68; P <0.0001). PFS advantage for DARA was consistent across most subgroups. However, a prespecified analysis showed significant interaction with ind/cons treatment arm (P< 0.0001). PFS HR for DARA vs OBS was 0.32 (95% CI 0.23–0.46) in the VTd arm and 1.02 (0.71–1.47) in the D-VTd arm. Median TTP was NR for DARA vs 46.7 mo for OBS (HR 0.49; 95% CI 0.38–0.62; P <0.0001). More pts in the DARA vs OBS arm achieved ≥CR (72.9% vs 60.8%; OR 2.17; 95% CI 1.54–3.07; P <0.0001). MRD negativity (in ≥CR pts at 10^-5) was 58.6% with DARA vs 47.1% with OBS (OR 1.80; 95% CI 1.33–2.43; P= 0.0001). Median OS was NR in either arm. Most common (≥2.5%) grade 3/4 adverse events (AEs) with DARA vs OBS were pneumonia (2.5% vs 1.4%), lymphopenia (3.6% vs 1.8%), and hypertension (3.0% vs 1.6%). Serious AEs occurred in 22.7% (DARA) vs 18.9% (OBS) of pts; the most common (≥2.5%) was pneumonia (2.5% vs 1.6%). 13 (3.0%) pts discontinued DARA due to an AE. The rate of infusion-related reactions was 54.5% (DARA-naïve pts) and 2.2% (prior DARA pts); 90% were grade 1/2.Second primary malignancies occurred in 5.5% (DARA) vs 2.7% (OBS) of pts. Conclusions: CASSIOPEIA part 2 demonstrated a clinical benefit of DARA maintenance in TE NDMM pts, with significantly longer PFS for DARA vs OBS. With current follow-up, maintenance PFS benefit appeared only in pts treated with VTd as ind/cons. Pts who received D-VTd ind/cons with or without DARA maintenance achieved similar PFS; longer follow-up is needed for PFS2 and OS. DARA significantly increased deeper response and MRD negativity rates vs OBS, and was well tolerated with no new safety signals. Clinical trial information: NCT02541383